Method for rapid reduction of iron oxide in a rotary hearth furnace

ABSTRACT

A method and apparatus for producing direct reduced iron from dry compacts composed of iron oxide and carbonaceous material by feeding compacts no more than two layers deep onto a hearth and removing all the volatiles and metallizing the compacts by exposing said compacts to a radiant heat source at a temperature of from about 2400° to about 2600° F (1316°-1427° C.) for a total time period of about four to ten minutes and partially cooling the compacts while discharging them from the hearth.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for achievingrapid and efficient reduction of iron oxide in a rotary hearth furnace.

BACKGROUND OF THE INVENTION

All previous patents and literature covering direct reduction of ironoxide in a rotary hearth (Heat Fast, Inmetco and Zia) have incorporateda low to medium temperature (below 2400° F. or 1316° C.) preheat zone inthe rotary hearth furnace (hereinafter also referred to as RHF) to dryand devolatize the pellets in order to avoid pellet exfoliation. Thedisadvantage of this method is that it decreases productivity due to thelong time required for pellets to reach optimum reduction temperature.

DESCRIPTION OF THE PRIOR ART

Applicants are aware of the following U.S. Patents concerning rotaryhearth furnaces used in the direct reduction of iron ore.

    ______________________________________                                        U.S. Pat. No.                                                                          Inventor  Issue Date                                                                             Title                                             ______________________________________                                        5,186,741                                                                              Kotraba   02-16-94 DIRECT REDUCTION                                           et al.             PROCESS IN A ROTARY                                                           HEARTH FURNACE                                    4,701,214                                                                              Kaneko    10-20-87 METHOD OF PRODUCING                                        et al.             IRON USING ROTARY                                                             HEARTH AND                                                                    APPARATUS                                         4,676,741                                                                              Pargeter  01-30-87 RADIANTLY HEATED                                                              FURNACE                                           4,636,127                                                                              Olano et al.                                                                            01-13-87 CONVEYING SCREW FOR                                                           FURNACE                                           4,622,905                                                                              MacDougall                                                                              11-18-86 FURNACING                                                  et al.                                                               4,597,564                                                                              Hanewald  07-01-86 ROTARY HEARTH                                              et al.                                                               3,836,353                                                                              Holley    09-17-74 PELLET RECLAMATION                                                            PROCESS                                           3,452,972                                                                              Beggs     07-01-69 FURNACE HEARTH                                    3,443,931                                                                              Beggs et al.                                                                            05-13-69 PROCESS FOR MAKING                                                            METALLIZED PELLETS                                                            FROM IRON OXIDE                                                               CONTAINING MATERIAL                               ______________________________________                                    

Beggs U.S. Pat. No. 3,443,931, teaches a method of metallizing compactsof iron oxide containing a carbonaceous material. The compacts areformed, dried, and preindurated up to a temperature between 1600°-1800°F. The pellets are then rapidly heated by exposure to a radiant heatsource which produces an environment at a temperature between2300°-2600° F. for a sufficient time so that a liquidus phase is formedwithin the compacts. After the liquidus phase is formed, the compactstend to shrink and then are immediately chilled by exposure to a coldenvironment.

Beggs U.S. Pat. No. 3,452,972, teaches apparatus for a refractoryfurnace hearth having wustite (FeO) as a constituent thereof and themethod of making such a refractory hearth. The subject furnace hearthhas particular utility in the processing of iron oxide containingmaterial, and is able to support such material during the reductionthereof without being destroyed during the process.

Holley U.S. Pat. No. 3,836,353, teaches a method of recovering iron andoxide impurities from steel furnace dust in which the dust first ismixed with finely divided coke and then this mixture is pelletized. Thegreen pellets thus formed are deposited over a layer of burnt pellets ona rotary hearth which successively conveys the pellets first through adrying zone, then through an initial heating zone in which the pelletsare gradually raised to a temperature at which the coke starts to burn,then through a decontamination zone in which the pellet temperature israpidly raised to a degree at which zinc, lead and sulfur impuritiesvaporize and in which these impurities are carried off and collected asoxides, and finally the pellets are carried through a reoxidation andhardening zone in which the temperature thereof is further increased toa sufficient degree and held for a long enough period of time to permitthe growth of grains of an oxide of iron on the surface of the pellets,thus to form hard bonded pellets which are not fused together.

Hanewald et al. U.S. Pat. No. 4,597,564, teaches a rotary hearth adaptedto rotate in horizontal plane having a top surface made of a loosegranular refractory material, advantageously dead burned dolomite grain.

MacDougall et al. U.S. Pat. No. 4,622,905, teaches an improvement infurnacing objects on the top surface of an impervious rotating hearth ina directly fired rotary hearth furnace by the use of fuel burning with aluminous flame e.g., coal.

Olano et al. U.S. Pat. No. 4,636,127, teaches a countercurrent fluidcooled conveying screw is disclosed. Suitable for furnace applications,the screw includes an outer shaft spatially circumscribing an innertube. A plurality of hollow, fluid cooled flights are affixed to theouter shaft and are in fluid flow communication with coolant coursingthrough the screw. The coolant is first directed through the flights andthen back through the outer shaft before exiting through the inner tube.

Pargeter U.S. Pat. No. 4,676,741, teaches a radiantly heated, travelinghearth furnace having a supplementary feed means positioned intermediatethe initial loading point and the final take-off point to increase thecapacity of the furnace for treating objects fed thereto. When theobjects are pellets of iron oxide and carbonaceous reductant theprovision of supplementary feed means about half-way along the travelpath of the hearth promotes uniformity of product by inhibition ofreoxidation of reduced iron by exposure to a fossil-fuel-fired furnaceatmosphere.

Kaneko et al. U.S. Pat. No. 4,701,214, teaches a method of producingiron from finely divided iron oxide comprising the steps of: mixing ironoxide or iron ore fines with finely divided coal and a binder to form amixture, agglomerating the mixture by compacting, pelletizing, orbriquetting the mixture to form agglomerates or pellets, introducing thepellets to a rotary hearth furnace to pre-reduce the iron in thepellets, introducing the pre-reduced pellets into a smelting reductionvessel as the metallic charge constituent, introducing particulatecarbonaceous fuel and oxygen to the smelting reduction vessel throughthe bottom of the vessel to react with the melt or bath within thevessel, reduce the iron to elemental iron and form an off gas containingCO and H₂, introducing the off-gas into the rotary hearth furnace asprocess gas to pre-reduce the pellets therein, and drawing off the hotmetal from the smelting reduction vessel.

The pre-reduced compacts are preferably discharged from the rotaryhearth furnace at a temperature of at least 1000° C. into the smeltingreduction vessel to form the molten iron product.

Kotraba et al. U.S. Pat. No. 5,186,741, teaches a pellet reclamationprocess includes forming green pellets of a mixture of steel furnacedust, a carbonaceous material such as coal, charcoal, lignite, petroleumcoke, or coke, and an organic binder. The green pellets are fed over alayer of burnt pellets on a rotary hearth furnace which successivelyconveys the pellets first through a drying and coking zone in which thepellets are dried and any volatile matter driven out of the carbonaceousmaterial. The pellets then travel through a reduction zone where thepellets are subjected to a higher temperature at which the containediron oxide is reduced and remains within the pellets and the zinc, leadand cadmium oxides are reduced, volatilized, re-oxidized and carried offas oxides in the waste gases. The reduced pellets (DRI) are ultimatelycarried into a discharge zone where they are discharged from the rotaryhearth furnace. An apparatus for performing the process is alsodisclosed.

SUMMARY OF THE INVENTION

This invention provides an improved method and apparatus for achievingrapid and efficient reduction of iron oxide in a rotary hearth furnace.Test results with this process, which will be known by the trade name ortrademark FASTMET™, show that properly formed pellets (dry compacts) canbe exposed immediately to a radiant heat source with a temperature of2400°-2600° F. (1316°-1427° C.) without causing exfoliation. Eliminatingthe low to medium temperature preheat zone and operating at highreduction temperature increases hearth productivity by 30 to 100%compared to other processes. In addition, energy efficiency can beimproved by burning most of the volatiles released from the compactsinside the rotary furnace, and by causing the compacts and products ofcombustion to flow co-currently in the first portion of the furnace andcounter-currently in the second portion of the furnace.

OBJECTS OF THE INVENTION

The principal object of the present invention is to provide an improvedmethod of achieving rapid and efficient reduction of iron oxide in arotary hearth furnace.

It is also an object of this invention to provide means for dividingrotary hearth gas flow into two portions rather than having the gasaccumulate and peak at the feed area where dust is most likely to beentrained.

Another object of the invention is to provide a low roof height in theinitial heating zone of a rotary hearth furnace to enhance the radiativeheat transfer to a layer of compacts on the hearth.

Another object of the invention is to provide a rotary hearth furnaceapparatus where the volatiles released from the compacts have a longerretention time, and can be more readily combusted.

Another object of the invention is to provide a rotary hearth furnacewith more efficient combustion than previously available, resulting in alower ultimate gas volume requiring gas cleaning.

A further object of the invention is to provide a rotary hearth furnacewhere the direction of the flue gas at the outlet is away from thehearth rather than sweeping across the hearth toward the side wall.

Another object of the invention is to provide a rotary hearth furnacewith a flue gas outlet of sufficient size to slow the gas velocityallowing entrained particles to fall back onto the hearth by gravity.

A further object of the invention is to provide a rotary hearth furnacewith improved atmosphere control at the hearth level to avoid oxidationof metallic iron.

Another object of the invention is to provide a rotary hearth furnaceapparatus for producing highly metallized iron having lower carboncontent.

Another object of the invention is to provide an improved rotary hearthfurnace in which energy efficiency is improved by using sensible heat inthe metallized compacts to preheat part of the fuel for the rotaryhearth furnace.

A further object of the invention is to provide a rotary hearth furnacecapable of operating with a very short retention time of 4 to 10minutes.

Another object of the invention is to provide a rotary hearth furnacewhich avoids any disturbance of the protective blanket of carbonmonoxide being evolved from the compacts in the final stages ofreduction.

Another object of the invention is to provide a rotary hearth furnacewhich maintains at least 1 percent excess carbon in the metallizedcompacts.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects will become more readily apparent byreferring to the following detailed description and the appendeddrawings in which:

FIG. 1 is a schematic diagram of the process for an improved method ofachieving rapid and efficient reduction of iron oxide in a rotary hearthfurnace.

FIG. 2 is a cross sectional view of the improved rotary hearth furnace.

FIG. 3 is a top view of the improved rotary hearth furnace.

DETAILED DESCRIPTION

Referring now to the drawings, and particularly to FIG. 1, the inventedmethod and apparatus for achieving rapid and efficient reduction of ironoxide in a rotary hearth furnace includes feed bins 10, 12 and 14 whichcontain the raw materials for the process. Feed Bin 10 contains ironoxide materials 16 which are comprised of, but not limited to, finelydivided iron ore fines, concentrate, by-product iron oxide and steelmill waste. Feed Bin 12 contains carbonaceous materials 18 which arecomprised of, but not limited to, pulverized coal, coke breeze, char,anthracite, charcoal and petroleum coke. Feed Bin 14 contains bindermaterials 20 which are comprised of, but not limited to, organicbinders, bentonite, or hydrated lime.

Materials from the feed bins 10, 12 and 14 are mixed together in properproportions, in a mixing unit 22. This mixture 24 is sent to anagglomerating unit 26 which either pelletizes, briquettes, extrudes orcompacts mixture 24 into consolidated units 28 which are thentransported to a drying unit 30 and dried at approximately 250° F.(about 121° C.) to remove moisture.

The dry compacts 32 are fed into a rotary hearth furnace (RHF) 34through feed chute 102 and deposited on the solid hearth 36, FIG. 2, ina layer 38 one to two compacts deep. The compacts pass under a radiationbarrier 100 and are exposed to a radiant heat source 40 of 2400°-2600°F. (1316°-1427° C.) for a period of 4 to 10 minutes during which timethe volatiles and carbon monoxide are evolved from the compacts andcombusted inside the furnace and most of the iron oxide is reduced tometallic iron and iron carbide.

The impact of rapid heating and high reduction temperature on thereduction rate of dry compacts 28 containing a mixture of iron oxide andcarbonaceous material can be seen in the following table. The tests wereconducted in an electrically heated tube furnace having a nitrogenatmosphere. The dry compacts (made with a mixture of magnetiteconcentrate, low-volatile bituminous coal and binder), were placedinside the preheated tube furnace and removed at 2 minute intervals andanalyzed for total and metallic iron to develop a metallization (percentof total iron content in the form of metallic iron) versus time curve.

    ______________________________________                                        Radiant Heat Source   Time to Reach 93%                                       Temperature (°F.)                                                                     (°C.)                                                                         Metallization (in minutes)                              ______________________________________                                        2150           1177   More than 10                                            2250           1232   7.6                                                     2350           1288   6.4                                                     2450           1343   5.8                                                     ______________________________________                                    

The productivity (lb/h-ft²) in a rotary hearth furnace 34 for a givenfeed material and hearth loading is inversely proportional to theretention time. For example, a retention time of 5.8 minutes shouldresult in a productivity 31% higher than a retention time of 7.6minutes.

The impact of rapid heating in an oxygen rich atmosphere on thereduction rate of dry compacts containing a mixture of iron oxide 16 andcarbonaceous material 18 was determined by comparing results of one testconducted in a nitrogen atmosphere and a second test conducted in an airatmosphere for the first 2 minutes followed by a nitrogen atmosphere forthe remaining time. The same test procedures were used as mentionedabove. The radiant heat source temperature was kept constant at 2450° F.(1343° C.) in both tests. Results were similar when using dry compactsmade with a mixture of hematite concentrate, low-volatile bituminouscoal and binder.

Since the temperature is kept uniformly high throughout all the heatingzones of the furnace, it is not necessary to locate the flue duct nearthe feed end to take advantage of the sensible heat of the products ofcombustion. The flue gas temperature would be approximately the sameregardless of outlet location 42 on the RHF 34. Therefore, it ispossible to improve fuel efficiency, when using carbonaceous materialscontaining volatiles, by locating the flue gas outlet 42 at themid-section of the RHF 34, between the charging and discharginglocations. This results in the compacts and products of combustionflowing co-currently in the first portion 44 of the RHF andcounter-currently in the second portion 46 of the RHF.

The gas flow through the RHF 34 is divided into two portions 44 and 48rather than growing cumulatively and peaking at the feed area 102 of theRHF where dust is most likely to be entrained. This allows the height ofthe roof in the initial heating zone in the RHF 34 to be low due to thepassage of low gas volume through the zone, thus enhancing the radiativeheat transfer to the layer of compacts. Volatiles released from thecompacts have a longer retention time inside the RHF and can be morereadily combusted. The more efficient combustion inside the RHF lowersthe ultimate volume of gas requiring gas cleaning.

Locating the flue gas outlet 42 in the roof of the RHF 34 providesadditional advantages like the direction of the flue gas at the outletis away from the hearth rather than sweeping across the hearth towardthe side wall. The flue gas outlet 42 can be made sufficiently large indiameter to slow the gas velocity down, allowing entrained particles tofall back onto the hearth by gravity.

The high temperature radiant heat source 40 is initially generated byburning fuel. Burner fuel is provided from a source 50, the fuels usedare, without limitation natural gas, fuel oil, by-product gas andpulverized coal. This fuel is distributed to roof burners or wallmounted burners 52. Oxygen for combustion is supplied by preheated oroxygen enriched air 54. Additional preheated or oxygen enriched air issupplied to burn volatiles and CO evolved from the compacts. Efficientcombustion is achieved due to the high operating temperature, and thelonger retention time of volatiles and carbon monoxide inside thefurnace due to locating the flue gas outlet 42 at the mid-section of theRHF 34 instead of at the feed end of the RHF.

Operating with an oxidizing atmosphere and high temperature in the earlystage of heating and reduction causes the volatiles to ignite on or nearthe surface of the dry compacts forming a radiant flame which enhancesthe heat transfer to the compacts.

In the final stage of reduction, the atmosphere maintained inside thefurnace is overall oxidizing to metallic iron. This allows the burnersto operate more efficiently, resulting in lower fuel consumption and theflexibility to use fuels such as pulverized coal and fuel oil. Thereduced iron is protected from oxidation by: operating with a very shortretention time of 4 to 10 minutes; avoiding disturbance of theprotective blanket of carbon monoxide being evolved from the compacts inthe final stages of reduction; and maintaining at least 1 percent excesscarbon in the metallized compacts.

One method of partially cooling the metallized compacts is injecting acoolant on, or near, the compacts immediately prior to their, dischargefrom the rotary hearth furnace. This coolant can comprise natural gas,pulverized coal, fuel oil or by-product gas. The coolant will dissociateinto carbon and hydrogen. Some, or all, of the carbon may form carbonmonoxide by reacting with carbon dioxide and water vapor. Free carbondeposited on the surface of the compacts will add further protectionfrom oxidation. Reformed gases, carbon monoxide and hydrogen, provideadditional blanket protection from the oxidizing products of combustionabove the compacts. The dissociation and reforming of the coolantpartially cools the hot compacts, transferring the heat to the reformedgases which are allowed to flow upward in the rotary hearth furnace 34and are combusted.

The advantages of this method are: improved atmosphere control at thehearth level to avoid oxidation of metallic iron; highly metallized ironcan be produced having lower carbon content; energy efficiency isimproved by using sensible heat in the metallized compacts to preheatpart of the fuel for the rotary hearth furnace.

To assist in optimizing productivity and monitoring product quality, awater cooled gas sampling probe is installed inside the rotary hearthfurnace to collect gas samples less than one inch above the surface ofthe compacts just prior to discharge. As the metallization level of thecompacts approaches 90 to 95%, the rate of reduction begins to slow andthe amount of carbon monoxide evolved begins to decrease. By monitoringthe carbon monoxide and oxygen content of the gas at this location, itis possible to predict product quality prior to obtaining chemicalanalyses of the product. A high carbon monoxide level indicates thereduction rate is still high and product metallization may be low. Amedium level of carbon monoxide indicates the reduction rate has slowedand product metallization is high. A low carbon monoxide level and/orpresence of oxygen indicates the reduction rate has stopped and theproduct may be oxidized. Based on this knowledge, adjustments can bemade to hearth speed, loading, temperature and/or atmosphere asnecessary to maintain optimum productivity and product quality. Thespecific level of carbon monoxide and oxygen for the above threeconditions must be calibrated for each furnace condition and feed mix.

The metallized compacts are discharged from the hearth 36 via one ormore helical water-cooled screws 56. The discharge device also levelsthe hearth. The hearth 36 is solid, is made of about 4 inches of thematerial being processed, and has wustite as a major constituentthereof. In this regard, it is a self-healing hearth. Any cracks or pitswhich develop are automatically filled with fresh fines without concernfor buckling of the refractory underneath.

The temperature of the discharged product 58 is approximately 1650° to2200° F. (899° to 1204° C.). The product 58 can be hot charged into amelter 60, hot briquetted 62, or cooled 64 and stockpiled. If thedischarged product is to be sent to a melter 60, then it may be placedin a transfer can 66 as hot direct reduced iron. It may also bedesirable to send discharged product 58 to a briquetting press 68 forformation of hot briquetted iron. Alternatively discharged product 58can be sent to a rotary drum cooler 70 which produces cold directreduced iron.

The reduction gas 72 after leaving the RHF 34 enters a flue gasconditioner 74. Conditioned gas 76 is transferred to a heat exchanger 78which is also feed with combustion air 80 through fan 82. Heat Exchanger78 serves to warm combustion air 78 into preheated oxygen 54. After theconditioned flue gas 76 leaves the heat exchanger it is sent to theappropriate pollution control equipment 84. Pollution control equipmentis comprised of scrubbers, electrostatic precipitators, cyclones, andbag houses. Treated gas 86 is drawn out of the pollution controlequipment 84 by a fan 88 and delivered to a stack 90 for discharge tothe atmosphere 92.

The hearth is conventionally sealed to the hearth enclosure by a waterseal 106, as described in Beggs U.S. Pat. No. 3,452,972. The annularhearth is supported on wheeled members 108 which can be driven by anyconventional driving means, as shown in Beggs U.S. Pat. No. 3,452,972 orin Hanewald et al. U.S. Pat. No. 4,597,564.

SUMMARY OF THE ACHIEVEMENT OF THE OBJECTS OF THE INVENTION

From the foregoing, it is readily apparent that we have invented animproved method and apparatus for of achieving rapid and efficientreduction of iron oxide in a rotary hearth furnace. Advantages of thismethod are: improved atmosphere control at the hearth level to avoidoxidation of metallic iron; highly metallized iron can be producedhaving lower carbon content; energy efficiency is improved by usingsensible heat in the metallized compacts to preheat part of the fuel forthe rotary hearth furnace.

It is to be understood that the foregoing description and specificembodiments are merely illustrative of the best mode of the inventionand the principles thereof, and that various modifications and additionsmay be made to the apparatus by those skilled in the art, withoutdeparting from the spirit and scope of this invention, which istherefore understood to be limited only by the scope of the appendedclaims.

What is claimed is:
 1. A method for producing direct reduced iron fromdry compacts composed of iron oxide and carbonaceous material andcontaining volatile materials therein, comprising:feeding said compactsno more than two layers deep onto a hearth; removing all of the volatilematerials by exposing said compacts to a radiant heat source at atemperature of from about 2400° to about 2600° F. for a first period oftime of from one to three minutes and subjecting said compacts to anoxidizing atmosphere with sufficient free oxygen to burn most of thecombustible gases evolved from the compacts during said first period oftime, and form combusted gases; metallizing the compacts by exposingsaid compacts to a radiant heat source at a temperature of from about2400° to about 2600° F. in an atmosphere devoid of free oxygen for asecond period of time of three to nine minutes; and causing said gasesand said compacts to flow co-currently during said first period of time,and to flow counter-currently for said second period of time, and toform metallized iron product; and discharging said metallized ironproduct from the hearth.
 2. A method according to claim 1, furthercomprising partially cooling the compacts while discharging them fromthe hearth.
 3. A method according to claim 1, wherein said metalizediron product are exposed to an atmosphere that is oxidizing to metalliciron for the final three minutes of said second period of time, but areprotected by excess carbon, or a thin blanket of carbon monoxide and/orhydrogen.
 4. A method according to claim 1, further comprising partialcooling of said metalized iron product prior to discharge by injecting acoolant on, or near, said metalized iron product immediately prior tothe discharge of metalized iron product from the hearth.
 5. A methodaccording to claim 4, wherein said coolant is selected from the groupconsisting of natural gas, pulverized coal, fuel oil, and by-productgas.
 6. A method according to claim 1 wherein said iron oxide isselected from the group consisting of finely divided iron ores, ironoxide concentrates, by-product iron oxides, and steel mill wastes.
 7. Amethod according to claim 1, wherein said carbonaceous material isselected from the group consisting of coal, coke breeze, petroleum coke,char, and charcoal fines.
 8. A method according to claim 1, wherein theiron oxide and carbonaceous material in said compact are bonded togetherwith an organic binder.
 9. A method according to claim 1, wherein theenergy for said radiant heating source is at least partially provided bycombusting volatile materials and carbon monoxide emitted from saidcompacts.
 10. A method according to claim 1, wherein the energy for saidradiant heating source is at least partially provided by combusting fuelselected from the group consisting of natural gas, pulverized coal, fueloil and by-product gas.
 11. A method according to claim 1, whereinoxygen is introduced into the furnace to aid combustion, the oxygensource being selected from the group consisting of preheated air, oxygenand oxygen enriched air.
 12. A method according to claim 1, wherein theproportions of iron oxide and carbonaceous material in said compact arecontrolled to maintain a consistent fixed carbon to iron ratio.
 13. Amethod according to claim 1, further comprising discharging thepartially cooled metalized iron product from said hearth into a hottransfer bin and hot charging the metalized iron product into a meltingfurnace.
 14. A method according to claim 1, further comprisingdischarging the partially cooled metalized iron product from said hearthinto a briquetting press to produce hot briquetted iron.